Add vendor prefixes for old safari and flexbox.

[RhodesPeter]

• add -webkit-box-flex
• We need to get @Jasminpatel1 to test this on her iPad before we can close the issue.

Related #120

[codecov-io]

Codecov Report

Merging #174 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #174   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines           9      9           
  Branches        4      4           
=====================================
  Hits            9      9

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update d94e8ed...3bd0eed. Read the comment docs.